From the publication of U. Tietze, and Ch. Schenk, Halbleiterschaltungstechnik, second edition 1971, page 415 and page 305, Schmitt trigger circuits are known in which besides the normal input signal (switching signal) an additional signal is applied in order to change the point at which the circuit switches. This arrangement has the disadvantage that the output state of the Schmitt trigger changes for decreasing supply voltage even though no switching signal is applied to its input. Further it is of course known to use difference amplifiers for amplification of electronic signals. However, the known difference amplifiers have supply current sources which supply a constant current. This has the disadvantage that, for decreasing supply voltages, the operating point of the difference amplifier will decrease to lower values and therefore cause subsequent switching circuits (for example Schmitt trigger circuits) to switch in an uncontrolled manner during such decreases of the supply voltage.
It is an object of the present invention to maintain the output transistor of the Schmitt trigger circuit in a blocked state, independent of the supply voltage value. The blocked state is to be maintained even when the supply voltage decreases substantially below the nominal supply voltage as, for example, is the case when the supply voltage is disconnected from the system.